Final answer:
The variable 'w' representing the additional weight a tanker can carry, given it already holds 64,000 tons of its 80,000 tons capacity, is correctly measured in tons.
Step-by-step explanation:
If a tanker can carry up to 80,000 tons of cargo and currently has 64,000 tons loaded, the variable w representing the amount of additional weight the tanker can carry would logically be in units of tons. None of the other units provided in the options (bushels, metric yards, gallons) are appropriate for measuring the weight of the cargo. Therefore, the correct unit for w is tons.